There are known devices as shown, for example, in U.S. Pat. No. 2,750,804 for controlling the lifting of a stack of sheets for maintaining the stack at a correct level for feeding a sheet at a time from the top of the stack.
As shown in the reference device, there is provided a lifting jack for raising the stack in increments in response to a sensing device set to actuate the jack when the level of the stack falls below the correct feeding level. The sensing device comprises a sensing arm in contact with the top of the stack and a stop member. When the stack is at the correct level, the stop member is in a first position to prevent operation of the jack. When the stack falls below the correct level, the stop member is displaced to a second position to permit operation of the jack to lift the stack until it is raised to the correct level, at which time the stop member is again moved to the first position. With the stack at the correct level, the sheets are fed from the top of the stack by feed rollers.
Because the device of the foregoing patent provides for lifting the stack incrementally in a step-by-step manner, resulting in the top of the stack being positioned between a position in contact with the feed rollers and varying positions spaced from the feed rollers, the feed rollers do not always exert equal and uniform forces against the top sheet being fed. Thus, and particularly when the top of the stack is out of contact with the feed rollers, the sheet feeding operation could result in skewed feeding of the sheets, mis-register, doublefeeds, skip feeds and the like. Also, the apparatus comprises a substantial number of component parts which not only add to the cost of the device but likely also necessitate frequent adjustment, servicing and maintenance.